Various types of conventional separation techniques are used to separate magnetic material from non-magnetic material. For example, slurries containing both magnetic material and non-magnetic material are commonly processed by hydroseparators and flotation cells to separate the magnetic material from the nonmagnetic material. One problem associated with various separation techniques concerns the loss of fine magnetite particles in such processes, e.g., fine, high grade magnetite particles (i.e., having a diameter less than or equal to 25 μm or −500 mesh).
A hydroseparator is a concentration apparatus commonly used in taconite plants. It is generally used to treat cyclone overflow from, for example, rougher magnetic separation and may, for example, be followed by a finisher magnetic separation stage. In principle, a hydroseparator process is similar to a selective flocculation process. Magnetically flocculated slurry is fed to a hydroseparator, which is designed to operate in such a way that suspended fine gangue particles leave the hydroseparator in an overflow. A hydroseparator's effectiveness is typically affected by the delicate balance needed between the amount of gangue separated and magnetic iron losses.
Each plant generally has its own strategy for operating such separation devices. Some plants may be more concerned with magnetic iron recovery and, therefore, operate hydroseparators at low upward velocities, while for others, it may be more important to separate silicate bearing minerals as efficiently as possible using higher velocities, thereby compromising recovery. For example, 35% to 65% of silica in a fine (−25 μm or −500 mesh) size fraction may be separated using hydroseparators operating with upward velocities that may vary between 1.7 mm/sec and 3.2 mm/sec (0.07 inch/sec and 0.13 inch/sec). Higher velocities may provide more effective separation of fine silicate minerals, but at the same time may increase magnetic iron losses.
In principle, high magnetic iron losses could be prevented for a hydroseparator by applying a magnetic field to capture particles going into an overflow stream, while operating the hydroseparator efficiently at high upward velocities. This principle was tested by Roe (“The Magnetic Reflux Classifier”, Mining Engineering, 5(3):312–315, March (1953)), who used a laboratory classifier tube of 46 mm (1.8 inch) internal diameter with a magnetic field imposed using a DC electromagnet coil near the top of the tube. The flux density was varied at the internal surface of the tube wall. Roe reported that high silica middlings along with free silica particles could be removed by careful control of the magnetic field and water supply. While an electromagnet may be used conveniently in a laboratory separator, its use in commercial separators of large diameter (e.g., 5–15 m or 15–50 ft) may pose various problems. For example, it is difficult to provide a strong enough magnetic field at the middle or center of such large separators with an electromagnet that surrounds the outer perimeter thereof.
In many circumstances, the concentration of magnetite in resultant magnetic material (e.g., material resulting from separation processes) must meet certain specifications. For example, current blast furnace practice (e.g., processing of magnetite) requires the silica content in taconite pellets to be in the neighborhood of 4%, and, for emerging technologies of direct reduction and direct smelting, even a lower silica content, e.g., less than 2%, may be desired.
In the processing of magnetic taconite, cationic flotation using, for example, flotation cells or columns, has been utilized to lower the silica content of magnetic concentrates. Size fractions coarser than 325 mesh become progressively higher in silica content in the form of locked siliceous gangue particles.
Efficiency is important when flotation is used as the last stage for concentration of ores when ores contain clay-type minerals. For example, fine slimes (e.g., those containing clay-type minerals) consume reagents used in such processes (e.g., for cationic flotation), such as primary amines, ether amines, and quaternary ammonium salts, leading to increased consumption of such reagents and decreased efficiency of flotation separation.
However, attempts to float coarse siliceous gangue by adding greater quantities of cationic collectors leads to an excessive loss of fine magnetite and, thereby, the iron recovery drops precipitously when the silica content in the flotation concentrates is lowered to below 4%. In the cationic silica flotation of magnetic taconite concentrates, iron losses are high due to simultaneous flotation of fine, well liberated, high-grade magnetite along with coarse middlings locked with magnetite.
Efforts have been made to develop more selective collectors and depressants to remove silica from magnetic taconite concentrates and minimize the flotation of fine, high-grade magnetite. However, various problems have occurred. For example, some reagents are not only expensive, but also may become an environmental concern in tailing ponds.
The use of a magnetic field to minimize magnetic material loss has also been reported in conjunction with flotation apparatus such as flotation columns. Its use is attractive not only because of lower cost, but also because of its limited effect on the environment.
For example, the use of a magnetic field in flotation was reported in conjunction with a copper sulfide ore for reducing the recovery of magnetic minerals (e.g., pyrrhotite and sulfide minerals locked with magnetite). The process used an electromagnet coil around a laboratory flotation column (Sonolikar et al., “Effect of magnetic field on column flotation of ore containing magnetic content”, Column Flotation '88, SME Annual Meeting, Phoenix, Ariz., Jan. 25–28, 1988). In laboratory-scale tests, the use of electromagnets may be convenient in selectively varying the field strengths. However, for commercial-scale equipment, the use of an electromagnet is impractical with respect to size, design, and safety.
Further, in Seetharama et al. (“Effect of magnetic fields in the flotation of magnetic concentrates”, Investigation into Production of Iron Ore Concentrates with Less Than 3 Percent Silica from Minnesota Taconites, Final Report to the State of Minnesota and the American Iron and Steel Institute, Mineral Resources Research Center, University of Minnesota, Minneapolis, Minn., 1991, 30 pages), a series of tests on magnetic taconite concentrates were carried out by applying magnetic fields to laboratory DENVER and WEMCO flotation cells.
In addition, Wu et al. (“The flotation of taconite in a magnetic field”, Proceedings, Minnesota Section SME 68th Annual Meeting, Center for Professional Development, University of Minnesota-Duluth, Duluth, Minn., 1995, pp. 245–256) tested the use of an electromagnet coil on a 203 mm (8-inch) diameter flotation column. Encouraged by preliminary test results, they extended the tests using permanent magnets around the flotation column and then in a 1.42 m3 (50-cu.ft.) WEMCO flotation cell. In these tests, 12.7 mm (½-inch) thick magnetic sheets were placed facing each other vertically in the direction of an axis through the center of the flotation column. An aluminum frame held the sheets in place.